Respiratory Medicine (2011) 105, 588e594
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/rmed
Clinical predictors of frequent exacerbations in
subjects with severe chronic obstructive pulmonary
disease (COPD)
Emily S. Wan a,b,*, Dawn L. DeMeo a,b, Craig P. Hersh a,b, Steven D. Shapiro c,
Richard A. Rosiello d, Susan R. Sama e, Anne L. Fuhlbrigge b,f,
Marilyn G. Foreman g, Edwin K. Silverman a,b
a
Channing Laboratory, Boston, MA 02115, USAh
Division of Pulmonary and Critical Care Medicine, Brigham & Womens Hospital, Boston, MA 02115, USA
c
Department of Medicine, University of Pittsburgh Medical Center, PA 15213, USA
d
Division of Pulmonary and Critical Care, Fallon Clinic, Worcester, MA 01605, USA
e
Department of Work Environment, University of Massachusetts Lowell, Lowell, MA 01854, USA
f
Harvard Vanguard Medical Associates, Boston, MA 02115, USA
g
Division of Pulmonary and Critical Care Medicine, Morehouse School of Medicine, Atlanta, GA 30310, USA
b
Received 13 August 2010; accepted 14 November 2010
Available online 10 December 2010
KEYWORDS
COPD acute
exacerbations;
COPD epidemiology;
Risk factors for acute
exacerbations of COPD
Summary
Background: Acute exacerbations are a significant source of morbidity and mortality associated with chronic obstructive pulmonary disease. Among individuals with COPD, some patients
suffer an inordinate number of exacerbations while others remain relatively protected. We
undertook a study to determine the clinical factors associated with "frequent exacerbator"
status within a population of subjects with severe COPD.
Methods: Case-control cohort recruited from two Boston-area practices. All subjects had GOLD
stage 3 or 4 (FEV1 50% predicted) COPD. "Frequent exacerbators" (n Z 192) had an average of
2 moderate-to-severe exacerbations per year while "non-exacerbators" (n Z 153) had no
exacerbations in the preceding 12 months. Multivariate logistic regression was performed to
determine the significant clinical predictors of "frequent exacerbator" status.
Results: Physician-diagnosed asthma was a significant predictor of frequent exacerbations. Within
a subset of our cohort, the modified Medical Research Council dyspnea score and FEF 25e75% predicted were also significant clinical predictors of frequent exacerbator status (p < 0.05). Differences in exacerbation frequency were not found to be due to increased current tobacco use or
decreased rates of maintenance medication use.
* Corresponding author. 181 Longwood Avenue, 4th floor, Boston, MA 02115, USA. Tel.: þ1 (617) 525 2112; fax: þ1 (617) 525 0958.
E-mail address: emily.wan@channing.harvard.edu (E.S. Wan).
h
Institution at which the work was performed.
0954-6111/$ - see front matter ª 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.rmed.2010.11.015
Clinical predictors of frequent exacerbations in severe COPD
589
Conclusions: Within our severe COPD cohort, a history of physician-diagnosed asthma was found to
be a significant clinical predictor of frequent exacerbations. Although traditional risk factors such
as decreased FEV1% predicted were not significantly associated with frequent exacerbator status,
lower mid-expiratory flow rates, as assessed by FEF 25e75% predicted, were significantly associated
with frequent exacerbations in a subset of our cohort.
ª 2010 Elsevier Ltd. All rights reserved.
Introduction
Chronic obstructive pulmonary disease (COPD) is a highly
prevalent disorder which is projected to become the fourth
leading cause of death globally by 2030.1 Acute exacerbations are a major source of the morbidity2,3 and mortality4
associated with the disease and are estimated to represent
35e60% of the total direct costs associated with COPD.5e12
Clinical features which have demonstrated association with
the development of acute exacerbations include lower
forced expiratory volume in the first second (FEV1) % predicted,2,13e17 increasing GOLD stage or BODE category,2,18,19 chronic cough20 and sputum production,13
advanced age,13,16,19 and clinical depression.17,21
The clinical observation that some patients with COPD
consistently experience a higher rate of exacerbations than
their peers despite having comparable reductions in FEV1
has led researchers to postulate the existence of a distinct
subgroup of "frequent exacerbators".22e24 Several studies
have demonstrated that the number of exacerbations from
year to year in a single subject is highly reproducible and
that a history of exacerbations predicts future exacerbations.15,23,25 Although exacerbation frequency generally
increases with declining lung function, recent work
suggests the “frequent exacerbator” phenotype remains
a distinct subgroup in all GOLD stages.23
Recent work has also brought attention to a subset of
patients who experience remarkably few exacerbations
despite significantly impaired lung function. This group of
“non-exacerbators” is likely systematically underrepresented and understudied given their less frequent indications for medical contact and frequent exclusion from
therapeutic trials.15,26 Careful characterization of both of
these extreme phenotypes within a cohort of severe COPD
subjects may offer additional insights into why certain
patients are prone to frequent exacerbations while others
remain relatively protected. We hypothesized that
frequent exacerbators would have more severe airflow
obstruction and a higher prevalence of respiratory symptoms including cough, phlegm, and dyspnea, than nonexacerbators.
Methods and materials
Study design and patient population
The study was designed as a cross sectional, case-control
cohort. Subjects were ambulatory patients between the ages
of 30e80 years old who were evaluated at two Boston-area
practices, Fallon Clinic and Harvard Vanguard Medical Associates. Subjects were enrolled from December 2006eOctober
2009. All subjects had 10 pack-year smoking history and
a diagnosis of severe COPD which was defined as GOLD Stage 3
or 4: a post-bronchodilator FEV1/FVC ratio of 0.7 and a postbronchodilator FEV1 of 50% predicted.27 Exclusion criteria
included pregnancy, a history of lung cancer, tuberculosis,
pulmonary fibrosis, asbestosis, organ transplantation, lung
volume reduction surgery or previous lung resection.
The protocol was approved by the Partners Institutional
Review Board (Partners Human Research Committee, 617424-4100) and written informed consent was obtained from all
participants.
Subjects were assessed at their baseline status, defined as
4 weeks since their most recent lower respiratory tract
infection (if any). Subjects were administered a modified
version of the standardized American Thoracic SocietyDivision of Lung Diseases Respiratory Epidemiology Questionnaire28 by trained study personnel. Spirometry was performed on an Easy One spirometer (ndd, Inc., Andover, MA,
USA) according to published guidelines29 both before and
approximately 20 min following the administration of inhaled
short acting bronchodilator (180 mcg albuterol by metered
dose inhaler through an Aerochamber spacer).
Variables and definitions
Acute exacerbations were defined as worsening symptoms
requiring treatment with systemic steroids (oral or parenteral) or antibiotics, a visit to the emergency room, and/or
admission to a hospital. Acute exacerbations in the previous
12 months were assessed by patient history. Patient reports
were verified by review of their medical records within the
last 24 months. All subjects were further classified as either
cases or controls based upon the following criteria:
“frequent exacerbators” reported an average of 2 exacerbations per year (either 2 exacerbations in the last 12
months or 4 exacerbations over the preceding 24 months
with at least one exacerbation in the last 12 months) while
“non-exacerbators” had no exacerbations over the last 12
months. All exacerbations were separated by 14 days
(reports occurring within 14 days of each other were
considered a single event).
Additional variables assessed during the study were
recorded as follows. Pack-years of smoking were calculated
as the average number of cigarettes per day divided by 20 and
then multiplied by the number of years smoked. Chronic
cough was considered present if the subject answered in the
affirmative to the question: “Do you usually cough like this on
most days for 3 consecutive months or more during the year?”
and answered 2 years to the question “For how many years
have you had this cough?”. Chronic phlegm was considered
present if the subject answered in the affirmative to the
question: “Do you usually bring up phlegm like this on most
590
E.S. Wan et al.
days for 3 consecutive months or more during the year?” and
answered 2 years to the question “For how many years have
you had trouble with phlegm?”. Chronic bronchitis was
defined as the presence of both chronic cough and chronic
sputum production. Exposure to a dusty job was considered
present if the subject answered affirmatively to the question: “Have you ever worked for a year or more in any dusty
job?”. Dyspnea was assessed using the modified Medical
Research Council (MMRC) questionnaire.30 Physician-diagnosed asthma (ever) was considered present if the subject
responded affirmatively to the questions “Have you ever had
asthma?” and “Was it confirmed by a doctor?”.
A list of each subject’s current medications was obtained
during the study visit e these lists were reviewed and
medication use in the following categories was recorded as
being present or absent: short or long acting beta agonists,
short or long acting muscarinic antagonists, inhaled corticosteroids, systemic steroids, theophylline, leukotriene
inhibitors, chronic home oxygen use, hydroxymethylglutaryl
CoA (HMG-CoA) reductase inhibitors (“statins”), aspirin,
diuretics, and additional cardiac medications (defined as
anti-hypertensive or anti-arrhythmic medications).
Statistical analysis
All analyses were performed using SAS version 9.1.3 (Cary,
NC, USA) on a SUN Unix system (SunOS 5.10, Santa Clara,
CA, USA). Univariate comparisons were performed using an
unpaired two-tailed Student’s t-test or Wilcoxon rank sum
test for normal and non-normally distributed variables,
respectively. Comparisons between binary and ordinal
variables were performed using Fisher’s exact test and the
Chi-squared test for trend respectively.
Stepwise multivariable logistic regression was performed
to identify the significant clinical predictors of “frequent
exacerbator” status. All variables with a univariate p-value
Table 1
<0.3 were considered candidates in the multivariate
regression; medication use variables were not included as
candidates due to concern for confounding by indication.
All analyses included adjustment for FEV1% predicted.
Candidate variables with a p-value 0.05 were considered
significant and were retained in the final model. Candidate
variables not retained using stepwise model-building were
re-introduced singly to assess for confounding (defined
a priori as 20% change in the effect estimate). Because
a significant proportion of subjects were missing the MMRC
dyspnea score,30 a separate multivariate analysis was performed on the subset of subjects with the MMRC score
available utilizing the same procedure as outlined above.
Results
Descriptive statistics and univariate comparisons between
frequent exacerbators and non-exacerbators are summarized in Table 1. There were no significant differences in
age, sex, or pack-years smoked between frequent exacerbators and non-exacerbators. Interestingly, despite
comparable mean FEV1% predicted, frequent exacerbators
had significantly lower maximal mid-expiratory flow rates
as assessed by FEF 25e75% predicted. A trend towards
increased current smoking was observed in non-exacerbators (p Z 0.07). Frequent exacerbators reported more
physician-diagnosed asthma and had higher MMRC scores
than non-exacerbators. Although the MMRC score was not
available in 22.6% of the subjects because many subjects
indicated that they were disabled from walking by conditions other than heart or lung disease, the missing rate was
similar between frequent exacerbators (22.9%) and nonexacerbators (22.2%).
Previous reports have noted that prior exacerbations are
strongly associated with the risk of future exacerbations.20,23
Even within our frequent exacerbator group, there was
Characteristics of study subjects.
Total Number of Subjects
Age
Male (%)
Non-white race (%)
Did not complete high school (%)
BMI
Pack-years
Current smoker
Chronic cough
Chronic sputum
FEV1% predicteda
FVC % predicteda
FEV/FVC ratioa
FEF 25e75% predicteda
MMRCb,c
Physician-diagnosed asthma (ever)
Frequent exacerbators
Non-exacerbators
p-value
192
68 (8.4)
43.2
5.7
21.9
28.3 (6.4)
60.8 (32.6)
18.9
50.0
47.9
34.7 (8.8)
63.9 (15.8)
0.42 (0.11)
15.1 (6.5)
2.6 (0.8)
36.5
153
69.3 (8.1)
49.7
3.3
17.7
28.8 (7.1)
64.0 (36)
27.5
43.1
50.3
35.9 (8.6)
64.4 (14.7)
0.43 (0.11)
16.1 (6.1)
2.3 (1.1)
25.5
e
0.14
0.28
0.31
0.35
0.48
0.55
0.07
0.23
0.67
0.20
0.78
0.47
0.05
0.02
0.04
Data are presented as percent or mean (SD).
a
Lung function variables reported are post-bronchodilator values.
b
Modified Medical Research Council Dyspnea scale (0e4, 4 representing severe shortness of breath).
c
22.6% subjects did not have MRC score available.
Clinical predictors of frequent exacerbations in severe COPD
Table 2
591
Medication use.
Inhaled corticosteroids (ICS)
Short acting beta agonists (SABA)
Short acting muscarinic antagonists (SAMA)
Long acting beta agonists (LABA)
Long acting muscarinic antagonists (LAMA)
Leukotriene inhibitor
Theophylline
Home oxygen use (current)
Chronic oral steroids
HMG-COA Reductase Inhibitor (“statin”)
Diuretic
Aspirin
Anti-hypertensive or anti-arrhythmic medications
Frequent
Non-exacerbators
p-value
83.4
89.3
47.1
58.8
43.9
8.6
3.7
5.9
18.8
41.7
41.2
34.2
57.2
54.7
79.3
36.7
38.7
29.3
1.3
1.3
2.0
3.9
41.3
37.3
39.3
58.7
<0.0001
0.0100
0.06
0.0003
0.0067
0.0029
0.31
0.10
<0.0001
1.0
0.50
0.36
0.82
Data are presented as percent.
evidence for an association between previous exacerbation
frequency and future exacerbation risk. The correlation
between the number of exacerbations in this group 0e12
months before enrollment and 12e24 months before enrollment was statistically significant (Rho Z 0.24, p-value
0.0004).
The rates of medication use and exposure to environmental
variables are summarized in Table 2 and Supplementary Table
A respectively. Frequent exacerbators demonstrated significantly higher rates of long acting bronchodilators and inhaled
and systemic steroid use e this likely represents confounding
by indication. There were no significant differences in the
rates of non-pulmonary medication use (aspirin, diuretics,
HMG-CoA reductase inhibitors, anti-hypertensive or antiarrhythmic medications). Similarly, there were no significant
differences in the measured environmental exposures
between frequent and non-exacerbators.
In the multivariate model including all subjects, a history
of physician-diagnosed asthma was a significant predictor of
frequent exacerbator status (Table 3). In the subgroup with
MMRC score available, MMRC score, post-bronchodilator FEF
25e75% predicted, and physician-diagnosed asthma were
significant predictors of frequent exacerbator status in the
multivariate model (Table 4). There were no significant
confounders in either of the final models.
Discussion
COPD exacerbations are a major cause of morbidity and
mortality. Recent reports in the medical literature support
the long-held clinical notion that COPD subjects vary widely
in their susceptibility towards acute exacerbations. The
Table 3
existence and characterization of “frequent exacerbators”
and relatively resistant “non-exacerbators” in a recent large
observational study has challenged the association of traditional risk factors with acute exacerbations.23 The findings
from our study support the existence of these distinct COPD
phenotypes and introduce new plausible risk factors.
The major finding in the recently published ECLIPSE cohort
study was the description of stable sub-phenotypes relating to
exacerbation susceptibility which appear to be independent of
lung function impairment.23 Thus, although low FEV1 has been
well established as a risk factor for acute exacerbations,13,14
its utility may be limited to comparisons between COPD
subjects with extremely disparate levels of FEV1 impairment
or of different GOLD stages. The disassociation of airflow
obstruction with exacerbation frequency is echoed in our
cohort in that no significant difference in mean FEV1% predicted was noted between our frequent and non-exacerbator
groups. Likewise, despite inclusion in our multivariate models,
FEV1% predicted was not found to be a significant predictor of
frequent exacerbator status. Additional findings which
support the concept of these sub-phenotypes as independent
phenomena include the lack of differences in age, gender, or
rates of chronic cough or sputum between frequent exacerbators and non-exacerbators. The lack of association with
these traditional risk factors may be attributable to the fact
our cohort has been enriched with these extreme phenotypes.
Two additional apparent paradoxes regarding the rates
of current smoking and medication use in our cohort
deserve discussion. First, the trend towards increased rates
of current smoking noted in our non-exacerbator group is
not an isolated event e similar observations have been
described in other studies.2,13,16,23,31 We believe this
reflects a “healthy smoker effect” (whereby subjects who
Multivariate model for frequent exacerbator status.
Variable
Unadjusted OR [95% CI]
p-value
Adjusted OR [95% CI]
p-value
Physician-diagnosed asthma
FEV1% predicteda,b
1.68 [1.05e2.68]
0.85 [0.67e1.09]
0.03
0.20
1.76 [1.09e2.83]
0.82 [0.64e1.06]
0.02
0.13
a
Post-bronchodilator value.
Data are reported as per 10% change in predicted value. Non-significant variables tested but not retained in final model include postbronchodilator FEF 25e75% predicted, age, wheezing, history of hypertension, maternal history of COPD, and sex. FEV1% predicted was
force included into the model.
b
592
Table 4
E.S. Wan et al.
Multivariate model for frequent exacerbations in subgroup with MMRC score.
Variable
a,b
FEF 25e75% predicted
MMRC score
Physician-diagnosed asthma
FEV1% predicteda,b
Unadjusted OR [95% CI]
p-value
Adjusted OR [95% CI]
p-value
0.6 [0.39e0.91]
1.46 [1.13e1.89]
1.61 [0.94e2.74]
0.77 [0.58e1.03]
0.02
0.004
0.08
0.08
0.53
1.50
2.05
1.08
0.04
0.003
0.01
0.71
[0.28e0.98]
[1.15e1.97]
[1.16e3.64]
[0.71e1.64]
MMRC Z modified Medical Research Council. n Z 267 (148 frequent/119 non-exacerbators).
a
Post-bronchodilator values.
b
Data are reported as per 10% change in predicted value. Non-significant variables tested but not retained in the final model include
current smoking, age, wheezing, chronic cough, history of hypertension, maternal history of COPD, and sex. FEV1% predicted was force
included into the model.
are frequently ill are more likely to quit smoking) rather
than a biologically protective effect of smoking. A similar
statement can be made regarding the significantly higher
rates of maintenance medication use among frequent
exacerbators. The efficacy of bronchodilators and inhaled
and systemic steroids in the treatment and prevention of
acute exacerbations has been studied previously.32e36 Our
results suggest that certain patients with COPD will
continue to suffer frequent exacerbations despite aggressive medical maintenance therapy.
In our cohort, self-reported physician-diagnosed asthma
was a significant clinical predictor of frequent exacerbator
status. The interpretation of this finding can be challenging.
First, subjects with asthma and COPD often report a formal
diagnosis of both diseases e studies have suggested an
overlap rate between 15 and 34%.37e43 Whether this degree
of overlap represents a true biological or pathophysiological
entity, as outlined by the Dutch hypothesis,44 or some degree
of misclassification remains unresolved. Even in studies that
employ rigorous measures such as bronchodilator reversibility
testing or methacholine challenge, differentiating between
or establishing the co-existence of COPD and asthma remains
challenging; subjects with asthma may not demonstrate
complete, immediate reversibility39,45 and a significant
proportion of COPD subjects will demonstrate some degree of
BDR41,46 and a positive response to methacholine challenge.47
In our cohort, despite the high self-reported rates of physician-diagnosed asthma, the rates of bronchodilator responsiveness and asthma diagnosed before age 18 were low and
did not vary by frequent/non-exacerbator status. Furthermore, although significantly more subjects in the frequent
exacerbator group reported a diagnosis of asthma, a greater
change from baseline FEV1 with bronchodilator medication
was observed in the non-exacerbator group (Supplementary
Table B).
Regardless of whether true biological overlap exists, the
significance of physician-diagnosed asthma as a risk factor
for exacerbations is plausible. COPD subjects with a history
of physician-diagnosed asthma report more respiratory
symptoms,48 worse health status,42 and are at increased
risk of requiring emergency room services or hospitalization.37,38,42 In the United States, COPD subjects with
a concurrent diagnosis of asthma have significantly
increased respiratory related costs.37,38 Thus, the term
“physician-diagnosed asthma” may capture an aspect of
more symptomatic or severe disease not well quantified by
lung function or other traditional risk factors.
Within the subgroup with MMRC dyspnea scores available,
several additional clinical predictors of exacerbations were
identified, including post-bronchodilator FEF 25e75% predicted
and the MMRC score. The modest but significant difference in
FEF 25e75% predicted between cases and controls despite
a lack of difference in FEV1% predicted values may reflect
worse obstruction at the level of the smallest airways,49
perhaps beyond some critical threshold, in frequent exacerbators. The significance of the modified MRC dyspnea scale in
predicting exacerbations may be due in part to the continued
reliance on the subjective report of increased shortness of
breath in defining acute exacerbations e whether some
subjects perceive or are more likely to report dyspnea and
hence are greater likelihood to meet criteria for an acute
exacerbation is debatable. Regardless, the utility of the
modified MRC score in predicting frequent exacerbator status
beyond FEV1 alone is suggested by this subgroup analysis. The
generalizability of the association with the MMRC score is
limited by the high rate of missingness for the variable which
resulted from strict adherence to a skip pattern in the questionnaire after subjects reported a disability from walking
other than heart or lung disease. Post hoc review revealed
that the majority of subjects who did not have an MMRC score
skipped these questionnaire items due to orthopedic
complaints, with a minority of subjects opting out due to
vascular or neurological problems. Although there was no
difference in the rates of missingness between cases and
controls, differential missingness with regards to other variables (such as current smoking) limits this analysis.
We acknowledge several limitations to this study in
addition to the ones outlined above. The retrospective and
cross sectional nature of the cohort, as well as the reliance
upon patient reported exacerbations, predisposes our study
to recall bias and resultant misclassification bias with
regards to case/control status. The review of medical
records for the majority of the subjects to verify reported
exacerbations is an advantage of our study design e
correlation rates between subject reported exacerbations
and the medical record review were high (Rho Z 0.7, pvalue <0.0001). Though the requirement for severe airflow
obstruction adds to the uniqueness of our cohort, it also
limits the generalizability of our findings. In addition, while
we did not directly assess for potentially confounding comorbid conditions such as heart failure, the non-differential rates of use of cardiac medications such as diuretics,
anti-hypertensive and anti-arrhythmic medications argues
against differential rates between frequent and non-
Clinical predictors of frequent exacerbations in severe COPD
exacerbators. Lastly, the modest size of our cohort may
limit detection of clinical variables with less profound
effect sizes (i.e. subject us to false negatives).
Despite these limitations, our study suggests that physician-diagnosed asthma is a significant clinical predictor of
frequent exacerbator status in our cohort. Significant
differences in FEV1% predicted, suboptimal medical
management, and increased rates of current tobacco use
were not the primary causes of frequent exacerbations in our
severe COPD subjects. Additional anatomical, environmental, or genetic factors may account for differences in
exacerbation frequency phenotypes. Future studies should
investigate the role of inflammatory markers and genetic
polymorphisms on the risk of frequent exacerbations.
Acknowledgments
E.S.Wan was involved in data analysis and manuscript
preparation. E.K.S. and S.D.S. were involved in concept and
design, funding, and manuscript editing. S.A.S. was
involved in the concept and design and manuscript editing.
D.L.D. and C.P.H. were involved in statistical support and
manuscript editing. R.A.R., A.L.F., and M.G.F. were
involved in data collection and manuscript editing.
We thank Eric Schwinder, Anne McDonald, R.N., and Katy
Allain R.N. for their work in data collection and management.
Conflict of interest
Dr. Shapiro has served on an advisory board for Boehringer
Ingelheim. Dr. Sama received grant funding for research
from Merck. Dr Fuhlbrigge has served is a consultant for the
design and analysis of epidemiologic studies, received
investigator initiated, unrestricted research support,
served on an advisory board and been a member of
a speakers bureau for GSK and Merck. She has also received
research funding from the NIH for studies related to asthma
and COPD. Dr. Silverman received grant support and
consulting fees from GlaxoSmithKline for studies of COPD
genetics. Dr. Silverman has also received honoraria and
consulting fees from AstraZeneca.
Drs. Wan, Hersh, DeMeo, Rosiello, and Foreman have no
conflicts to disclose.
Supplementary material
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.rmed.
2010.11.015
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